Cyclic welding behavior of covalent adaptable network polymers

Surface welding effect of covalent adaptable network (CAN) polymers enables self-healing, reprocessing and recycling of thermosets, but little is known about their welding behaviors during repeated welding-peeling cycles. In this article, we study the cyclic welding effect of an epoxy based thermal-sensitive CAN. Surface roughness is generated by rubbing the sample on sandpapers with different grid sizes. The welding-peeling cycles are repeated on the same pair of samples for five times, with roughness amplitude and interfacial fracture energy measured in each cycle. It is shown that the roughness gradually decreases during the repeated welding cycles, especially when a long welding time or high welding pressure is applied. Even though lower roughness amplitude promotes the contact area, the interfacial fracture energy reduces due to the increased BER activation energy after long-time heating. A multiscale constitutive model is adopted, where we incorporate an explicit expression of interfacial contact area as a function of root-mean-square roughness parameter. The model is able to capture the evolving interfacial fracture energy during repeated welding cycles by using the measured roughness parameter, network modulus and BER activation energy. The study provides theoretical basis for the design and applications of CANs involving cyclic welding-peeling operations. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 402-413.